The present invention relates generally to obtaining a silica film with a low dielectric constant that is stable over time with improved methods of dehydroxylation of hydroxylated silica film.
As used herein, the term xe2x80x9csilicaxe2x80x9d means a compound having silicon (Si) and oxygen (O), and possibly additional elements.
Further, as used herein, xe2x80x9cmesoporousxe2x80x9d refers to a size range which is greater than 1 nanometer, but significantly less than a micrometer.
Porous silica films are potentially useful as low dielectric constant intermetal materials in semiconductor devices, as low dielectric constant coatings on fibers and other structures, and in catalytic supports.
Most of the U.S. semiconductor industry has successfully implemented the use of interlevel dielectric films with dielectric constant (kxe2x80x2) in the range of 2.6 to 3.0. Further reductions in dielectric constant could improve the operating speed of semiconductor devices and reduce overall device cost by decreasing the number of metallization levels that are required.
Porous silica films with nanometer-scale porosity that are produced from solution precursors may be classified into two types (1) xe2x80x9caerogel or xerogelxe2x80x9d films in which a random porosity is introduced by controlled removal of an alcohol-type solvent, and (2) xe2x80x9cmesoporousxe2x80x9d surfactant-templated films in which the pores are formed by removal of the surfactant. The formation of xe2x80x9cmesoporousxe2x80x9d films is based on the principle of using surfactants to template mesoporosity. In the surfactant-templated films, the pores either form ordered (e.g. hexagonal) arrays or disordered structures, with the characteristic pore diameter being defined by the surfactant micelle size. xe2x80x9cMesoporousxe2x80x9d refers to a size range which is greater than 1 nm, but significantly less than a micrometer. In general, this refers most often to a size range from just over 1.0 nm (10 angstroms) to a few tens of nanometers.
In order to obtain low dielectric constant in highly porous silica films it is necessary to minimize the number of hydroxyl groups in the structure, especially at pore surfaces. Without dehydroxylation, the dielectric constant of porous silica films can exceed to a considerable extent, that of dense silica (i.e. approximately 4.0).
Porous silica materials need to be treated at very high temperatures of over 800xc2x0 C. in order to obtain silica surfaces that are highly dehydroxylated. For microelectronic applications, semiconductor devices with dielectric films and metal lines cannot usually be processed over about 500xc2x0 C. Thus, porous silica films such as mesoporous films used in microelectronics are still partially hydroxylated after heat treatment in the temperature range of 450xc2x0 C. to 500xc2x0 C.
E. F. Vansant, P. Van der Voort and K. C. Vrancken, in Characterization and Chemical Modification of the Silica Surface, Vol. 93 of Studies in Surface Science and Catalysis, Elsevier, New York, N.Y. (1995), and C. J. Brinker and G. W. Scherer, in Sol-Gel Science, Academic Press, New York, N.Y. (1990), have reviewed dehydroxylation of silica surfaces by fluorination or by treatment with silane solutions. Furthermore, low dielectric constants in aerogel-type films have been demonstrated by both (a) fluorination treatment, and (b) a two-step dehydroxylation method of (1) initial silane solution treatment (e.g. trimethylchlorosilane or hexamethyidisilazane (HMDS) in a solvent), and then (2) following this solution treatment with a treatment in hydrogen-containing gases (e.g. 10% hydrogen in nitrogen) at moderately high temperatures of 300-450xc2x0 C. The silane/forming gas(H2 in N2) treatment is discussed in U.S. Pat. No. 5,504,042 and some of the other related patents by Smith and colleagues that are referenced therein.
Heretofore, the most successful demonstration of low dielectric constant films with dielectric constant of 2.5 or less has been with aerogel type porous silica films. However, the smallest average pore diameter typically possible in spin coated aerogel films falls in the size range of 10-100 nm. Also, low dielectric constant mesoporous films prepared from solutions must be prepared by techniques such as spin coating which can be used in a manufacturing process line. Over large-area circular wafers, other coating techniques such as dip coating are not as convenient as spin-coating, and uniform film thickness by dip coating over the entire wafer is difficult and has not been reported in prior literature for either dip coating or spin coating.
Although dehydroxylation of silica films, especially porous silica films made by the aerogel process has been reported to achieve low dielectric constants, there is still a need for yet lower dielectric constant films.
Hence, there is a need for a mesoporous film having a low dielectric constant that is stable over time.
It is therefore an object of the present invention to provide a method of dehydroxylating a silica surface that is hydroxylated in order to obtain a low dielectric constant.
It is further an object of the present invention to provide methods of making mesoporous silica film, which result in low dielectric constant and permit spin-coating techniques that do not require atmosphere controls. These methods of making mesoporous silica film further provide for superior control of film thickness and average pore diameter smaller than 5 nm. The present invention differs from the aerogel method through the use of specific surfactant molecules to template porosity in spin-coated films.
The present invention is a method of dehydroxylating a silica surface that is hydroxylated having the steps of exposing the silica surface separately to a silicon organic compound and a dehydroxylating gas. The silicon organic compound is also known as a silylation agent. Exposure to the silicon organic compound can be in liquid, gas or solution phase, and exposure to a dehydroxylating gas is typically at elevated temperatures. The present invention has the advantages of providing a mesoporous film of a silica material having low dielectric constant that is stable over time. Further advantages of the present invention potentially include improved safety and lower cost when applied on a large scale.
In one embodiment, the improvement of the dehydroxylation procedure is the repetition of the soaking and dehydroxylating gas exposure. In another embodiment, the improvement is the use of an inert gas that is substantially free of hydrogen.
In yet another embodiment, the present invention is the combination of the two-step dehydroxylation method with a surfactant templating method of making a mesoporous film. The method of making a mesoporous silica film has the general steps as described in U.S. Pat. No. 5,922,299 hereby incorporated by reference, viz: combining a surfactant in a silica precursor solution, spin-coating a film, and heating the film to remove the surfactant to form a mesoporous film that is at least partially hydroxylated, and dehydroxylating the hydroxylated film to obtain the mesoporous film. According to the present invention, the improvement comprises dehydroxylation of the hydroxylated film with the two-step dehydroxylation of exposing the hydroxylated film separately to a silicon organic compount and to a dehydroxylating gas to obtain a mesoporous film with low dielectric constant.
Further improvements are realized for any of the above referenced embodiments by using a polyoxyethylene ether compound as the surfactant.
It is advantageous that the small polyoxyethylene ether surfactants used in spin-coated films as described in the present invention will result in fine pores smaller than about 5 nm. Most often the average pore diameter can be tailored with surfactants in the size range from about 2 to about 5 nm. This average pore diameter range is desirable in interlevel dielectric films that separate metallization lines in semiconductor devices to minimize diffusion of metal species during repeated heat treatments. These small polyoxyethylene ether surfactants are different from large polyalkylene oxide xe2x80x9cblock co-polymerxe2x80x9d surfactants used to make fibers with pores larger than 5 nm. Further advantages of the present invention include a method which provides for superior control of film thickness and thickness uniformity across a coated wafer, films with low dielectric constant, and spin-coating techniques which do not require atmosphere controls.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.